Measurement techniques to determine the aberration of an optical system, by obtaining through-focus intensity images that are produced when the object is a point source at infinity, are shown. The analysis of the aberrations is made using the extended version of the Nijboer-Zernike diffraction theory. This theory provides a semi analytical solution of the Debye diffraction integral and thus a direct relation between the intensity distribution of the field at the focal region and the exit pupil of the optical system